1. Field of the Invention
The present invention generally relates to a driving circuit, and more particularly, to a light source apparatus and a driving circuit thereof, adapted for achieving a driving current which increases and decreases with a same speed.
2. Description of Related Art
Nowadays, because of the progresses made in the optoelectronic field, light emitting diodes (LEDs) having the advantages of lower cost and convenience in operation are more widely applied. For example, single color LEDs are often used for traffic signs, handheld illumination devices, and instrument lights, while multi-color LEDs are more often used in advertising boards and ultra large displays.
With respect to a display, in order to turn on/off a driving current of a conventional driving circuit of LED, it is often required to change the input reference voltage of the operation amplifier (OP-AMP) or power off the OP-AMP. In this case, the operation current of an inner chip drastically varies.
FIG. 1 is a schematic diagram illustrating a circuit of a conventional light source apparatus. Referring to FIG. 1, in a light source apparatus 100, an LED series D1 through Dn is coupled between a system voltage Vdd and a drain of a transistor M1. A current I is a driving current for the LED series D1 through Dn. According to the electric characteristic of the OP-AMP 101 (i.e., when only one terminal is inputted with a voltage, the voltage at the positive input terminal is equal to the voltage at the negative input terminal), the current value of the driving current I is Vin/R (i.e., the voltage value of the driving voltage Vin divided by the resistance value of the resistor R). Therefore, the amount of the driving current I can be determined by the driving voltage Vin. In other words, the driving current I can be turned off or turned on by adjusting the driving voltage Vin. Further, the driving current I can also be turned off by powering off the OP-AMP 101 for lowering the gate voltage of the transistor M1 to a low voltage level. However, all approaches discussed above for turning off the driving current I cause drastic variation of the operation current of the inner chip. Further, when turning off the driving current I by adjusting the driving voltage Vin, it is required to precisely control the voltage Vin outputted from the previous stage (i.e., light source controller 102). Otherwise, the driving current I will increase and decrease with different speeds.